gen tartrate, the bismuth is partially eluted even in 0.02 M hydrochloric acid (see Figure 4).This is evidenced by the reduced bismuth peak on elution with 0.5 M hydrochloric acid. It is likely that this decrease results from the competition of the tartrate with the resin 1igand.for the bismuth(II1). As shown by the last chromatogram in Figure 4, this problem is solved by adding 0.3 to 0.4 g of a copper salt to all samples containing hydrochloric acid.
ACKNOWLEDGMENT The authors thank Rohm and Haas for a gift of the XAD-4 resin used in the synthesis, and John J. Richard who performed the sulfur analyses.
LITERATURE CITED (1) Yu Zolotov. "Oraanic Sulfur Reaaents", Seminar Iowa State University, Oct. 1975. (2) F. E. Beamish, Talanta, 14, 991 (1967). (3) E. E. Rakorskii and M. II Starozhitskaya, Zh Anal. Khim., 29, 2094 (1974). (4) R. F. Propistosova and S. B. Savvin, Zh. AnalKhim., 29, 2097 (1974). (5) L. A. Demina, 0. M. Petrukhim, and Yu. Zolotov, Zh. Anal. Khim., 25, 1463 119701 - -, (6) E. Bayer, Angew. Chem. 73, 659 (1961). (7) F. H. Pollard, G. Nickless, K. Burton, and J. Hubbard, Microchem. J., I O , 131 (1966). %
(8) A. Lewqndowski and W. Szezepaniak, Fresenius' Z.Anal. Chem., 202, 321 (19641. (9) E. Eayer, H: Fiedler. L. Hock, Dotterbach,G. Schenk, and V. Voelter, Angew. Chem., 76, 76 (1964). (IO) H. J. Kramer and B. Neidhart, Radiochem.-Rad. 22, 209 (1975). (11) D. E. Leyden and G. H. Luttrell, Anal. Chem., 47, 1612 (1975). (12) J. F. Dingman, K. M. Gloss, E. A. Milano. and S. Siggla, Anal. Chem., 46, 774 (1974). (13) P. Heizmann and K. Ballschmiter, Fresenius'Z. Anal. Chem., 266, 206 (1973). (14) G. Koster and G. Schmuckler, Anal. Chlm. Acta, 38, 179 (1967). (15) J. S. Fritz, and W. G. Millen, Talanta, 18, 323 (1971). (16) G. V. Myasoedova, 0. P. Eliseeva, S. B. Savvin, and N. I. Uryanskaya, Zh. Anal. Khlm.. 27, 2004 (1972). (17) G. V. Myasoedova, L. I. Bol'shakova, 0. P. Shroeva, and S. B. Savvin, J. Anal. Chem. USSR, 28, 1382 (1973). (18) Albert Zlatkis, W. Bruening, and E. Bayer, Anal. Chem., 41, 1692 (1969). (19) R. Hering, K. Trennl, and P. Neske, J. Pract. Chem., 32, 291 (1966). (20) J. S. Fritz, R. K. Giliette, and H. E. Mishmash, Anal. Chem., 38, 1669 (1966). (21) M. D. Seymour and J. S. Fritz, Anal. Chem., 45, 1394 (1973). (22) J. S. Fritz and L. Goodkin. Anal. Chem., 46, 959 (1974). (23) M. D. Arguello and J. S. Fritz, future publlcation. (24) S. M. Ahrned and B. J. P. Whailey, Fuel, 51, 190 (1972). (25) L. Goodkin, M. D. Seymour, and J. S. Fritz, Talanta, 22, 245 (1975).
RECEIVEDfor review February 17, 1976. Accepted April 8, 1976. Work performed for the U S . Energy Research and Development Administration under Contract No. W-7405eng-82.
Exchange Rates and Water Content of a Cation Exchange Membrane in Aprotic Solvents Maria Lopez, Brian Kipling, and Howard L. Yeager* Department of Chemistry, The University of Calgary, Calgary, Alberta T2N 1N4, Canada
The properties of a perfluorinatedsulfonic acid ion exchange membrane (Nafion) in the solvents water, acetonitrile, and propylene carbonate are described. Equilibrium solvent compositions within the membrane phase for sodium and cesium ion forms have been determined using different membrane pretreatments. The membranes may be effectlvely dehydrated in nonaqueous media without prior heating. Rates of exchange of sodium and cesium ions for hydrogen ions decrease markedly in these solvents.
Several authors have investigated potential analytical applications of commercially available ion exchange membrane materials. Blaedel and co-workers (1-4) have demonstrated that these materials are highly permselective and can function as effective preconcentrating devices; a corresponding analytical application has been reported ( 5 ) .Extensive use of ion exchange membranes in electrochemical systems has been made in both aqueous and nonaqueous media (6-10). Nafion-brand sulfonated fluorocarbon cation exchange membranes are chemically inert, highly permeable to cations, permselective, and are available in a variety of forms (8, 11-13). The material is based on a perfluorinated ethylene backbone with pendant side chains of the form
where n < 20. A recent study suggests that Nafion contains fewer impurity sites capable of binding metal ions than some other commonly used ion exchange membranes ( 4 ) .We are 1120
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
interested in applying these membranes to nonaqueous systems and have chosen to study the properties of Nafion in the solvents propylene carbonate (PC) and acetonitrile (AN). These solvents were chosen because of their general suitability in electrochemical applications. We have performed tracer diffusion studies of alkali metal ions in Nafion for P C and water systems to determine membrane diffusion coefficients ( 1 4 ) .Although satisfactory results were obtained in aqueous experiments, the nonaqueous results showed scatter. A recent study of the physical properties of Nafion suggests that the water content of the membrane may be a significant factor in the morphology of the polymer (15). T o clarify significant factors controlling ion transport in Nafion, it is important to characterize membrane composition under various experimental conditions. We report studies of solvent uptake and water content of Nafion as a function of counter ion and pretreatment procedure in PC and AN. In addition, the rates of ion exchange in each solvent have been surveyed.
EXPERIMENTAL Materials. Ion exchange membranes were H-form Nafion-125 (Plastics Dept., Du Pont and Co.), with nominal capacity and thickness of 0.83 mequiv/g and 0.013 cm, respectively. Capacity measurements conducted on these membranes in water-equilibrated form yielded a value of 0.87 mequiv/g. Capacities of various samples of membrane used in this study were calculated from this value and the respective membrane densities. Densities were determined from the measured dimensions of weighed membrane portions. Sodium iodide (Fisher Certified) and cesium iodide (Alfa Ventron, 99.9%) were used without further purification. The purification and analysis of propylene carbonate has been described previously (16). Water content of PC used in this study was found to be 5 X M
WAVENUMBER
(cm-')
Flgure 1. Infrared spectra of aqueous Nafion in H-form (lower trace) and Na-form (upper trace)
by Karl Fischer titration. Acetonitrile (Fisher Certified) was predried by stirring over calcium hydride and then distilled on a spinning band distillation apparatus (Perkin-Elmer Corp., 4 5 theoretical plates), collecting the middle 75%. Water content of the purified product by M. Karl Fischer titration was found to be 7 X Infrared Spectra. Spectra of membrane samples were recorded using a Perkin-Elmer 337 grating infrared spectrophotometer. Membrane samples with low water contents were checked for moisture pickup by remeasuring the spectra several times. Negligible pickup resulted if spectra were recorded within 5 min of removal from sealed containers. Procedure. Membranes were converted to alkali metal ion forms by immersion in successive portions of alkali metal iodide solutions of the appropriate solvent in glass stoppered flasks. Concentrations of equilibrating solutions varied from 1M for aqueous sodium iodide to 0.03 M for cesium iodide in acetonitrile,depending on the solubility for each system. After complete exchange,the membranes were immersed in anhydrous solvent for two weeks to ensure swelling equilibrium. Other membranes were first exchanged in aqueous solution, dried, and then immersed in AN or PC for 2 weeks as well. The water contents of all samples were determined by an infrared method to be described. Membrane thicknesses were measured with a disc type micrometer (L. S. Starrett Co., Athol, Mass.). AN or PC content was determined by measuring weight changes, correcting for water variation and change of counterion where necessary. Studies of ion exchange rates were repeated at several solution concentrations. As expected, salt concentration had no effect on the rates of ion exchange (17).
Membrane samples in alkali metal ion form were dried by heating in vacuum at a maximum temperature of 220 "C; higher temperatures caused discoloration.An infrared method was used to monitor water contents for these samples. The Karl Fischer method of water determination was found to be inapplicable because of slow desorption of water from the membrane.
RESULTS AND DISCUSSION The infrared spectra of Nafion in the as-received hydrogen form and after exchange for sodium ion in aqueous solution are shown in Figure 1. The membrane in the H-form is virtually opaque throughout the infrared region, probably because of absorption by species such as H502+ and H904+(18). The spectrum of the Na-form exhibits three main peaks: OH stretch of water (3530 cm-l), CF stretch (2350 cm-l) from the polymer backbone, and the water scissor mode (1630 cm-l). Figure 2 shows additional solvent bands which appear when the Na-form is equilibrated with anhydrous PC; these include the C H stretch (3000,2940 cm-l) and carbonyl stretch (1800 cm-'). All membranes studied were essentially opaque in the region from 1350 to 500 cm-l. Spectra of Cs-form membranes showed identical peaks to corresponding materials in the Na-form. The 3530 cm-l band was used to calculate water molarities in various samples of Nafion, using a molar absorptivity of 37 1. mol-l cm-l (19) and the measured membrane thickness. This value had been obtained for water dissolved in hydro-
WAVENUMBER
(ern.')
Figure 2. Infrared spectrum of PC equilibrated Nafion, Na-form
time
(minutes)
Figure 3. Dehydration of Nafion as a function of counterion and drying temperature
Na': 110 O C (O),220 O C (A); : ' s C
110 OC (0). 220 O C (A)
carbon solvents, however. Therefore, this method was tested by comparing results to those obtained from drying-weight loss experiments, with good agreement. The large change in membrane transmittance at 2700 cm-l which accompanies conversion of Nafion from the H-form to alkali metal ion forms was used to monitor the rate of ion exchange. The results of dehydration tests for membranes exchanged in water are shown in Figure 3. The Cs-form loses water a t a much faster rate than does the Na-form. However, the final residual water contents for both forms were about the same for sufficient drying times, less than 1h a t 220 "C and up t o 4 h a t 110 "C. The residual water levels were 0.12 M for the Na-form and 0.08 M for the Cs-form, corresponding to about 1water molecule per 10 exchange sites. The drying procedure used in successive experiments consisted of overnight heating at 200 OC in vacuum to ensure low residual water. The effect of this drying procedure on membrane composition after subsequent solvent equilibration is shown in Table I. Results are listed for two treatment procedures; one in which the as-received membrane is directly exchanged in PC or AN, and another in which prior aqueous exchange and drying is followed by equilibration in the solvent. Acetonitrile contents were more difficult to reproduce because of the high vapor pressure of this solvent. The dried membranes generally show similar or lower solvent uptake, particularly for PC. This may be due to the larger molecular volume of PC over AN. Similar solvent size effects have been shown for the swelling of ion exchange resins by nonaqueous solvents (20). In addition, resins with higher cross-linking have been found to swell more after dehydration than similar resins of lower cross-linking, ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1121
Table I. Effect of Drying on Nafion Solvent Composition
Table 11. Times for 90% Exchange of Nafion, €I-form Time, h
Cation-solvent Na+-PC
cs+-PC Na+-AN &+-AN
Treatment Exchanged in PC Exchanged in H20, dried, PC Exchanged in PC Exchanged in H20, dried, PC Exchanged in AN Exchanged in H20, dried, AN Exchanged in AN Exchanged in H20, dried, AN
mol HzO/ mol -SO3-
mol solvent/ mol -SO3-
0.3 0.3
1.9 1.2
0.3
2.0 0.3
0.1
0.4
0.3
1 2
0 0.1
1 1
presumably because of the prevention of polymer-polymer interactions by the cross-links (21, 22). Since Nafion would be expected to possess little or no cross-linking, it is perhaps not surprising that lowered solvent uptake is observed after drying. This lowered uptake for the dried material is shown in aqueous experiments as well. An as-received membrane exchanged for sodium ion has a water-exchange site mole ratio of 11:l while a predried Na-form membrane yields a corresponding ratio of 7:l when soaked in water. Recent evidence suggests that Nafion exhibits an open, structured form, perhaps due to the presence of large groups of exchange sites formed into ionic clusters (15). Dynamic mechanical and dielectric studies of Nafion showed a large dependence of these properties on water content (15).In the dry state, Nafion may condense into a form which allows extensive polymer-polymer interactions, preventing solvent swelling. Final water contents are similar for the two treatments, as shown in Table I. The water content of the as-received membrane is about 2 mol of water per mol of exchange sites. Exchange in anhydrous solvent is therefore as efficient as predrying the membranes for dehydration. Water removal may be facilitated by the ion exchange process, with associated higher solvent uptake due to the more open structure of the membrane over a predried sample. Some water pickup is observed when the dried membranes are equilibrated with solvent, especially with acetonitrile. Although more stringent precautions may avoid this pickup, it is evident that the dried materials are initially hygroscopic. Therefore, it may be difficult to generate completely anhydrous membranes in routine nonaqueous applications. This pickup is more pronounced for the sodium counterion, indicating that ionic hydration is an important factor. The results of exchange experiments performed on asreceived Nafion in the H-form are shown in Table 11, where times to reach 90% exchange for sodium and cesium counterions are listed. The rates of exchange parallel the solvent uptake values, with higher solvent uptakes generating faster exchange. The large difference between sodium and cesium ion exchange rates in water becomes smaller for the aprotic solvents. Two important factors governing these rates are the selectivity of the ion exchange membrane and the diffusion
1122
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
Solvent
Na+
cs+
H2O AN PC
0.03 5.8 19
47
0.67 91
coefficient of the alkali metal ion in the membrane phase for each solvent system (17). Since the ratio of cesium to sodium ion exchange rate varies considerably in Table 11, the relative contributions of these two factors may be solvent dependent. The basicity of anionic exchan-- groups may be dramatically altered in aprotic media. For example, the ion pair association constants for sodium and cesium trifluoroacetates in PC are 200 and 18,respectively (23).If this trend is similar in solvent equilibrated membranes, it would be revealed in measured selectivity coefficients. Measurements are under way to determine selectivity coefficients and self-diffusion coefficients for alkali metal ions in Nafion in order to reveal the importance of each factor on these ion exchange rates. The much faster exchange rate for sodium over cesium ion in aqueous systems may be attributed to different water contents in the two ionic forms; the water-exchange site mole ratio in the sodium form is 11:1, while for cesium ion the corresponding ratio is 3:l.
ACKNOWLEDGMENT Appreciation is expressed for free samples of membrane materials from the E. I. du Pont de Nemours and Co.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
W. J Biaedel and T. J. Haupert, Anal. Chem., 38, 1305 (1966). W. J. Biaedel and E. L. Christensen, Anal. Chem., 39, 1262 (1967). W. J. Blaedel and T. R . Kissel, Anal. Chem., 44, 2109 (1972). W. J. Blaedel and R. A. Niemann, Anal. Chem., 47, 1455 (1975). G. L. Lundquist, G. Washinger. and J. A. Cox, Anal. Chem., 47,319 (1975). H. Lund and P. E. Iverson, in "Organic Electrochemistry", M . M. Baizer, Ed., Marcel Dekker, New York, N.Y., 1973, Chapter IV. F. W. Dampier, J. Appl. Electrochem.,3, 169 (1973). D. J. Vaughan, Du Pont Innovation, 4 (3), 10 (1973). B. Kratochvil and K. R. Betty, J. Electrochem. Soc., 121, 851 (1974). J. E. Harrar and R. J. Sherry, Anal. Chem., 47, 601 (1975). W. G. F. Grot, G. E. Munn, and P. N. Walmsley, paper presented at The Electrochemical Society Meeting, Houston, Texas, May 1972. W. G F. Grot, Chem.-lng.-Tech.,44, 167 (1972). M. F. Hoover and G. B. Butler, J. Polym. Sci., Part C, 45, 1 (1974). Unpublished results. S. C. Yeo and A. Eisenberg, Polym. Prep., Am. Chem. SOC.,Div. Polym. Chem., 16, 104 (1975). H. L. Yeager, J. D. Fedyk, and R. J. Parker, J. Phys. Chem.,77, 2407 (1973). F. Helfferich, "Ion Exchange", McGraw-Hill, New York, N.Y., 1962, Chapter 6. G. Zundel, "Hydration and Intermolecular Association", Academic Press, New York, N.Y., 1969. J. W. Forbes, Anal. Chem., 34, 1125 (1962). Y. Marcus in "ion Exchange and Solvent Extraction", VoI. 4, J. A. Marlnsky and Y. Marcus, Ed., Marcel Dekker, New York, N.Y., 1973 B. R Sundheim, M. H. Waxman, and H. P. Gregor, J. Phys. Chem., 57,974 (1953). R. Arnoldand S. C. Churms, J. Chem. Soc., 1965, 325. M. L. Jansen and H. L. Yeager, J. Phys. Chem., 78, 1380 (1974).
RECEIVEDfor review December 24,1975. Accepted April 6, 1976. This work was supported by the National Research Council of Canada and the University of Calgary.
